1. Field of the Invention
The present invention relates to a flue gas treatment system that reduces mercury in flue gas discharged from a boiler, and to a method for removing mercury in flue gas.
2. Description of the Related Art
Conventionally, various systems for removing mercury in flue gas have been considered because, for example, coal combustion boilers serving as combustion apparatuses in power stations and other facilities discharge flue gas containing highly toxic mercury.
A coal combustion boiler generally includes a wet desulfurization apparatus for removing sulfur content in flue gas. In flue gas treatment facilities including such a boiler provided with a desulfurization apparatus that serves a flue gas treatment apparatus, as chlorine (Cl) content increases in flue gas, the ratio of water-soluble divalent metallic mercury increases. This facilitates collection of the mercury with the desulfurization apparatus, as is well known.
In recent years, various proposals have been made on denitration apparatuses that reduce NOx, and on methods and apparatuses for treating the metallic mercury in combination with a wet desulfurization apparatus that uses an alkali absorbing solution as SOx absorbent.
As treatment of metallic mercury in flue gas, removal methods using an absorbent such as activated carbon or a selenium filter have been known. These methods, however, require special means for absorption removal, and are not suitable for treatment of a large volume of flue gas, such as flue gas in power stations. As a method for treating metallic mercury in a large volume of flue gas, methods have been proposed that include: gas-atomizing a chlorinating agent in a flue at an upstream process in a high-temperature denitration apparatus, oxidizing (chlorinating) the mercury on a denitration catalyst to be aqueous hydrogen chloride, and then absorbing it in a downstream wet desulfurization apparatus (for example, see Japanese Patent Application Laid-open No. 10-230137 and Japanese Patent No. 3935547). Apparatuses and techniques for spraying gas in a flue have been commercialized as NH3 spray devices of denitration apparatuses, and the same means can be used for gas atomization of a chlorinating agent.
FIG. 7 depicts an example of a conventional mercury removal system. As shown in FIG. 7, a related-art flue gas treatment system 100 includes: a denitration apparatus 13 that reduces nitrogen oxide in flue gas 12 discharged from a coal combustion boiler 11 that supplies coal as a fuel F, and that sprays vaporized hydrogen chloride 23 into the gas to oxidize mercury; an air heater 14 that recovers heat in gas from which the nitrogen oxide has been reduced; a dust collector 15 that reduces dust in gas from which the heat has been collected; a desulfurization apparatus 16 that reduces sulfur oxide in gas from which the dust has been reduced; a smoke-stack 17 from which desulfurized gas is discharged to the outside; and a hydrogen chloride vaporizer 21 that produces the vaporized hydrogen chloride 23. In FIG. 7, indicated by reference numerals 41 and 42 are mercury monitors, 43 is an redox potential measurement controller (an oxidation-reduction potential (ORP) controller), 45 is gypsum slurry containing mercury discharged from the desulfurization apparatus, 46 is a belt filter, 47 is gypsum, 60 is a lime feeder, and 61 is lime (particles or slurry).
A proposal has been made of supplying, instead of spraying hydrogen chloride into flue gas, chlorine compounds during combustion in the boiler so as to produce hydrogen chloride (Japanese Patent No. 3698916).
When employing the mercury removal systems that spray hydrogen chloride or the like as proposed in Japanese Patent Application Laid-open No. 10-230137 and Japanese Patent No. 3935547, 35% hydrochloric acid is evaporated in the hydrogen chloride vaporizer. Accordingly, a large amount of concentrated hydrochloric acid is required, and the resulting by-product, i.e., dilute hydrochloric acid, is discharged by an amount equivalent to about 80% of the concentrated hydrochloric acid (35% hydrochloric acid). This poses a problem that the dilute hydrochloric acid results in a waste product for the power station.
The chlorine concentration varies depending on the type of coal used in the power station (i.e., coal type). Even when a coal type with high chloride concentration is used, a large amount of 35% concentrated hydrochloric acid is necessary, and dilute hydrochloric acid is discharged by an amount equivalent to 80% of the 35% concentrated hydrochloric acid. Thus, to establish recycling plants as ancillary facilities to treat the dilute hydrochloric acid as a waste product, a set of plants needs to be established. This increases costs for establishing and maintaining the plants.
It is also considered to establish recycling plants within power generating facilities. This requires, however, substantial addition in treatment processes because non volatile materials are circulated and concentrated in the treatment of the dilute hydrochloric acid.
In the proposal of Japanese Patent No. 3698916, hydrogen chloride is produced by combustion in the boiler. The efficiency of converting chlorine compounds to hydrogen chloride in the boiler varies depending on the combustion in the boiler, and therefore is not constant. In addition, the mercury concentration in flue gas is not constant all the time. This poses a problem that hydrogen chloride cannot be supplied by a constant amount so that mercury is securely reduced as mercury chloride.
Thus, there is a demand to establish an inexpensive mercury removal system that disposes no dilute hydrochloric acid discharged as a by-product, and that can stably reduce mercury.
The present invention is made in view of the foregoing, and has an object to provide an air pollutant control system and a method for removing mercury in flue gas that reduce operating costs.